建筑学论文代写 The Special Properties Of Concrete
Admixtures are used to give special properties to fresh or hardened concrete. Admixtures may enhance the durability, workability and characteristics of a given concrete mixture. Admixtures are used to overcome difficult construction situations, such as hot or cold weather placements, pumping requirements. The use of admixtures to increase the workability or to accelerate the hardening of the concrete will be permitted when approved by the Engineer. Admixture dosages shall result in the mixture meeting the specified plastic and hardened properties.
The major reasons for using admixtures are
1. To reduce the cost of concrete construction.
2. To achieve certain properties in concrete more effectively than by other means.
3. To maintain the quality of concrete during the stages of mixing, transporting, placing, and curing in adverse weather conditions.
4. To overcome certain emergencies during concreting operations.
Set Retarding Admixtures
Set retarding admixtures are water soluble chemicals that have little or no other effect than to delay the setting of the cement. They do not plasticize significantly and have little or no effect on the water demand or other properties of the concrete. Set retarding water-reducing admixtures not only delay the setting of the cement but are also efficient in plasticizing concrete or reducing its water demand. Most commercially available retarders are of this type.
Retarding water-reducers and retarding high range water reducers are used to:
Give workability retention to the concrete
Delay the setting time of concrete
Prevent the formation of cold joints
Increase initial workability
Increase ultimate strength
Produce economies in mix designs
The main types of chemical used for retarding admixtures are:
Sucrose and other polysaccharides
Salts of boric acid
Salts of phosphoric, poly-phosphoric and phosphonic acids.
The main types of chemical used for retarding water reducing admixtures are:
Hydroxy carboxylic acid salts
Lignosulphonic acid salts
These may be also be used in conjunction with sulphonated naphthalene/melamine-formaldehyde condensates or polycarboxylates to produce retarding high range water reducing admixtures. The retarder molecule chemically adsorbs onto the cement particle in a mechanism similar to that described for water reducers. The main difference is the strength of the chemical bond that is formed. This strongly links the retarder molecule onto the cement surface, blocking and slowing down the rate of initial water penetration into the cement. Retarder molecules also chelate calcium ions in solution, slowing the crystallization of portlandite. These two mechanisms slow the growth of hydration products, delaying the stiffening and setting of the cement but once initial hydration starts, the retarder molecules are swamped and normal hydration proceeds.